Issue 13 - NEXRAD Radar Operations Center - NOAA

Don’t Restrict the KlystronThe present WSR-88D Transmitter Technical Manual(NWS EHB 6-511 paragraph 7.8.6.5.4.2 step 54)limits the maximum Pulse Forming Network (PFN)voltage to 5000 volts. Recent research in the area oftransmitter tuning has revealed that this is an unnecessarilyrestrictive requirement. Here’s why:• The most restrictive requirement in the Modulatorand Klystron is the maximum Beam Voltage (epy)spec of the Klystron, Drawing Number 1213614,which states that a peak power output of 750 KWshall be obtained with an epy of 62KV max. ThePulse Transformer drawing, 156C742A01, specifiesa turns ratio step up of 28:1. The epy spec of62KV max then results in a primary voltage of2214 volts peak. The primary voltage divided by100 is available at 3A12TP2 (modulator). Incidentally,the Pulse Transformer permits an epy of65KV max.NEXRADNowHow does this relate to PFN voltage? Transmissionline theory states that if a delay line (i.e., the PFN) ischarged and a switch (i.e., the RBDT stack) is closed, ifthe load impedance (looking into the pulse transformerprimary) is exactly equal to the source impedance (i.e.,the PFN characteristic impedance) exactly ½ of thecharged voltage will appear across the load (Primary ofpulse transformer) until the line is discharged with noover or undershoots. In practice, the perfectly matchedcondition is NOT realized so that the primary voltage is43% of the PFN voltage just prior to closing the ReverseBlocking Diode Thyristor (RBDT) switch and there areundershoots and/or overshoots. This can be seen in Figure1. Channel 1 is the PFN voltage at 3A12TP1. Itindicates a PFN voltage of 4930 volts. Channel 2 is thePulse Transformer primary voltage. This indicates avoltage of 21.20 X 100 = 2120 volts peak. This isequivalent to an epy of 2120 X 28 = 59.4 KV.(Continued on Page 14)Figure 1:Voltageovershootsandundershoots.page 13